1. Field of the Invention
The present invention relates to the general field of illumination. More specifically, the present invention relates to illumination of areas containing potentially explosive agents which can be in the form of dust, vapor or gas.
2. Prior Art
Illumination of an area containing a potentially explosive agent can be complicated, particularly if the area is quite large and/or a variety of potentially explosive agents may be present. An example of such an area is a hanger for painting commercial aircraft, although similar problems are present in various other areas classified as hazardous.
One approach has been simply to avoid placement of electric lighting equipment in a very hazardous area (Class 1, Division 1, in the terminology of the National Electric Code) which can be achieved by isolating the area containing the potentially explosive agent from the lighting equipment. For example, light can be beamed through windows or transparent panels from a nonhazardous location. In a large open area, the lighting equipment may be placed at a location remote from the source of the potentially explosive agent with increased ventilation to decrease the possibility of ignition or explosion.
If the electric lighting equipment must be placed in the very hazardous area, the conventional solution is to provide lighting fixtures classified as "explosion-proof". Examples of such fixtures are shown in Appleton et al. U.S. Pat. No. 3,675,007, issued July 4, 1972, and Lowndes U.S. Pat. No. 4,142,179, issued Feb. 27, 1979. Such fixtures are not intended to be airtight. Rather, conventional explosion-proof lighting fixtures are designed and located so that the maximum temperature of all exposed parts of the fixtures does not exceed 80 percent of the ignition temperature of the explosive agent present and, recognizing that a malfunction may occur, such fixtures are designed to withstand an internal explosion and allow pressure to be released, such as by escape of burned gases, but at temperatures sufficiently low that external ignition is avoided.
Explosion-proof fixtures are very expensive and may require expensive maintenance and replacement if used in a very hazardous area. In addition, the limitation as to the external temperature still limits the areas in which the fixtures can be used.